The present invention is related to circuits in which a field-effect transistor device controls power transfer from an alternating polarity electrical power supply to a load means, particularly when such field-effect transistor devices are capable of being integrated in monolithic integrated circuits, or when a plurality of same are used in series.
Various solid state devices have been used in circuits as the primary means for controlling power transfer from an alternating polarity electrical power supply to whatever kind of load means is of interest for use in the circuit. For instance, planar bipolar power transistors have been used but these are devices which are not bidirectional by nature and which exhibit an inherent, more or less irreducible, minimum power dissipation characteristic even when fully switched on. And to be fully switched on, bipolar power transistors require a substantial amount of base current, i.e., control current, especially for higher collector, or load, currents. Furthermore, they are also subject by nature to thermal runaway.
Perhaps more commonly used for controlling alternating polarity power supplies are thyristors of various kinds such as silicon controlled rectifiers and triacs. Such thyristors are switching devices primarily used in alternating polarity power supply control circuits because of their capability for handling relatively large power dissipations when switched fully on, and for withstanding substantial reverse voltages when switched fully off. An advantage of these devices over bipolar power transistors is that they require little electrical power at device control gates whether operating in the off condition or in the on condition.
However, such thyristors also have several disadvantages such as being a latching switch, that is, operating only in fully on or fully off states. Further, thyristor devices can be switched off by sufficiently reducing the current therethrough, and can be switched on by sharp voltage transients thereacross--both results being obtained without any action taking place at the control terminal of the thyristor device. Hence, the control terminal of the thyristor has relatively little continuous control capability. This same control terminal, in many situations, cannot be electrically isolated simply and inexpensively from the load circuit and may require large triggering currents to switch on the thyristor device. Finally, a thyristor device cannot be easily provided in a monolithic integrated circuit with other circuit components because of its structure and power dissipation.
Hence, better primary power controlling devices are desired for use in controlling power transfer from alternating polarity electrical power supplies in alternating polarity operated circuits. Particularly useful would be a device which could be easily provided in a monolithic integrated circuit along with other circuit components, at least some of which would also be used in controlling power transfer from the alternating polarity power supply used. This would require that such a device not have too large a resistance if switched fully on, despite substantial current loads, but which would have a structure easily fabricated in such an integrated circuit. Further, the device should have a bidirectional current conduction capability for circuits in which current rectification is not desired.
Field-effect transistor devices can have many of the characteristics just described, including having a very symmetrical bidirectional current conducting capability when on. This is certainly so for metal-oxide-semiconductor field-effect transistor (MOSFET) devices which have the advantage of having the gates therein very well isolated from the channel regions of the device. This isolation aids in providing a circuit to operate the field-effect transistor device when both the circuit and these devices are formed in a monolithic integrated circuit chip, a difficult arrangement when the integrated circuit is to operate with an alternating polarity power supply. Such circuits must permit the operation of other circuit component devices in the monolithic integrated circuit while also controlling power transfers from the alternating polarity power supply through operating the primary power transfer control field-effect device.
Electronic component device theory shows that field-effect transistors are operated by controlling the voltage appearing between the gate thereof and the connection to that one of the two channel regions therein which is effectively serving as the transistor source. Difficulties arise in those circuits using a field-effect transistor to control power transfers from an alternating polarity power supply because the two connections to the channel region of such a transistor serve alternately as the source rather than one of them serving continually as the source.
Certain field-effect transistor structures are especially useful for providing a field-effect transistor capable of controlling power transfer from an alternating polarity power supply to a load means. Such transistors should have low channel resistance when operated fully on--on the order of tenths of ohms--if they are to be used successfully in a monolithic integrated circuit. Then these transistors, when passing several amps of current, will not cause the circuit to suffer heat dissipation sufficient to disrupt the operation of other circuit components. Further, this channel resistance in a fully turned on device should be more or less symmetrical so there are no current rectification effects occurring. And, of course, the transistor device structure should be capable, when switched off, of withstanding, without breakdown, voltages at least as large as the peak voltage provided by the alternating polarity supply used. In this regard, the applications referenced above entitled "Semiconductor Apparatus" teach various devices, effectively field-effect transistors, which exhibit one or more of these desired characteristics.
The application referenced above entitled "Alternating Polarity Power Supply Control Apparatus" by Pinckaers shows a circuit making use of such a field-effect transistor to control power transfer from an alternating polarity electrical power supply to a load means. A basic circuit from this application is shown in FIG. 1 of the present application. This circuit operates using an enhancement mode, p-channel, MOSFET, 10, for controlling power transfers from an alternating polarity electrical power supply, 11, to a load means, 12. As the above referenced Pinckaers application indicates, several other essentially equivalent circuit versions using other kinds of electronic components for transistor are also possible.
In operation, the two channel connection regions, 15 and 16, of transistor 10 alternately serve as source and drain depending on which one is positive with respect to the other in a cycle of the output voltage of supply 11. With switch, 20, open, the gate region, 14, transistor 10 will approximately be at the positive voltage appearing on either of the sides of supply 11 by virtue of one of the diodes, 17 or 18, thereby being forward biased (the other being reversed biased). This is because gate 14 is electrically isolated from the remaining portions of transistor 10 so that the voltage on gate 14 can never be more than a diode drop plus the negligible drop across the resistor 19, below the positive voltage value appearing on one or the other side of supply 11 at any time.
The voltage drop across either of the diodes 17 or 18 plus the voltage drop across resistor 19 with switch 20 open will always be less than the threshold voltage of transistor 10. Gate 14 will never be more than approximately a diode voltage drop away in voltage from the voltage appearing on whichever of terminating regions 15 and 16 is positive. Hence, with switch 20 open, device theory indicates that transistor 10 will always be switched off and hence there will be no power transfer from supply 11 to load means 12.
However, with switch 20 closed, a voltage will be impressed across resistor 19 by the constant polarity as to cause transistor 10 to switch on and will be added to the voltage drop occurring across whichever one of the diodes 17 and 18 is forward biased through being connected to the positive side of supply 11. Hence, for a sufficient supply 21 voltage, device theory indicates that transistor 10 will be on a times when switch 20 is closed.
A variant of FIG. 1 is shown in FIG. 2 where the diodes 17 and 18 in the circuit of FIG. 1 are provided in the circuit of FIG. 2 by the parasitic effective diodes inherent in the structure of transistor 10 as formed in a substrate having a connection point therein, 13. Several other effective, but parasitic, circuit components inherent in transistor 10 are also shown in FIG. 2 in "lumped" form, all of these being present as a result of the structure of transistor 10. These parasitic components are, in many cases, more likely to be significant for a power controlling transistor such as transistor 10 because it is usually of a relatively large physical size compared to transistors used for controlling signals only. The same component designations are used in the FIG. 2 circuit as are used in the FIG. 1 circuit where the same component is being employed.
Among the parasitic components shown in FIG. 2 are a gate-to-channel capacitance, 22, and a channel-to-substrate capacitance, 23. Two other parasitic capacitances, 8 and 9, are shown which are each effective between gate 14 and one of channel terminating regions 15 or 16. There is also shown the two parasitic channel-to-substrate diodes, 24 and 25, which in the FIG. 2 circuit variant are to take the place of diodes 17 and 18 in the FIG. 1 circuit. Diodes 24 and 25 are provided in the structure of transistor 10 by the semiconductor pn junctions occurring between the regions for source and drain connections in transistor 10 and the substrate of transistor 10. In a p-channel MOSFET, for instance, the substrate material is of n-type conductivity while the connection regions 15 and 16 which terminate the ends of the channel region in the transistor 10, and serve as source and drain regions therein, are formed by diffusion or implantation of p-type conductivity impurities into the substrate material.
Also associated with these pn junctions are parasitic resistances 26 and 27, and parasitic capacitances, 28 and 29. All of these parasitic components will have more or less of an effect in the operating behavior of transistor 10, and so in the behavior of the circuit in which transistor 10 is provided, the significance of the effect depending on the conditions existing in such a circuit. Of course, capacitance 22 is essential for switching on transistor 10 while the other parasitic components shown with transistor 10 are normally desired to contribute as insignificantly as possible to the circuit operation. In FIG. 2, of course, parasitic diodes 24 and 25 are also necessary for circuit operation since they are replacing diode 17 and 18 of FIG. 1.
To have diodes 25 and 26 substitute for diodes 17 and 18 in FIG. 1 effectively, a direct electrical connection is made between substrate 13 and resistor 19 as shown in FIG. 2. Assuming that frequencies of operation are not high enough so that the parasitic capacitance shown in FIG. 2 (which would also be present in connection with transistor 10 of FIG. 1) act to change the operating characteristics from those occurring in connection with the circuit operation of FIG. 1, the operation of the FIG. 2 circuit will proceed in the manner described for the operation of the FIG. 1 circuit. Further, other circuits more or less equivalent to that shown in FIG. 1 of the present application, as described in the Pinckaers application, can also be shown to have circuit equivalents of the nature shown in FIG. 2 here and these would operate similarly.
Even though the circuits of FIGS. 1 and 2 can control alternating polarity electrical power supply power transfer, a desirable feature for circuits for this purpose would be eliminating the need for a second power supply such as the constant polarity supplies used in the FIG. 1 and 2 circuits. In some circumstances, the use of another control circuit scheme which would avoid depending on the parasitic diodes inherent in the primary power transfer controlling transistor, but not requiring two separate diodes, would also be desirable. And in high voltage circuits, the voltage breakdown of a single field-effect transistor device used to control power transfers from the alternating polarity power supply may not be sufficient.